The decontamination and making fluid-tight of old water, gas, or sewer mains are often obtained by application, inside the conduit, of a fluid-tight sheath, of plastic, of resin, or by a lining of fabric coated with resin. This application may be made over several hundreds of meters without a break in continuity. The various inlets of secondary branches or of branches of buildings are then obstructed by the fluid-tight layer deposited. Piercing of said layer from the outside necessitates the opening of the roadway at each building inlet, the dismantling of the inlet, then the piercing of the sheath in order to restore the branch. These operations are extremely expensive and, in addition, pose numerous traffic problems when the branch to be restored is situated under a roadway.